Yield of excited CO molecules from dissociative recombination of HCO+ and HOC+ ions with electrons

The authors have investigated CO band emissions arising from the dissociative recombination of HCO(+) and HOC(+) ions with thermal electrons in a flowing afterglow plasma. The quantitative analysis of the band intensities showed that HCO(+) recombination forms the long-lived CO(a (3)Pi) state with a yield of 0.23+/-0.12, while HOC(+) recombination favors formation of CO(a' (3)Sigma(+)) and CO(d (3)Delta) with a combined yield of greater than 0.4. The observed vibrational distribution for the CO(a) state reproduces theoretical predictions quite well. The vibrational distributions for CO(a') and CO(d) are, in part, inverted, presumably as a consequence of a change in CO equilibrium bond length during recombination. The observations are compatible with current knowledge of the potential surfaces of states of HCO and HCO(+).     

Yield of electronically excited N2 molecules from the dissociative recombination of N2H+ with e-

Quantitative spectroscopic observations of the N2 first positive band system (N2(B 3Pig-A 3Sigmau+))/electron in a recombining N2H+ flowing-afterglow plasma indicate that a substantial fraction of the product N2 molecules are formed in one or more of the low-lying triplet states, B 3Pig, A 3Sigmau+, and W 3Deltau. The total measured N2(B-A) emission intensity from N2(B,v' > or = 1) is equivalent to a yield of (19 +/- 8)%. The effect of rapid collision-induced transitions between states of the triplet manifold is discussed..     

The role of electronically excited states in recombination reactions

Methods are described for including the participation of bound electronically excited states in calculations on radical recombination reactions. These methods are illustrated by applying them to the reactions For O₂, accurate ab initio potentials are used in calculations which show that the electronic degeneracy and long-range part of the potential are likely to be crucial in determining the contribution of a given electronic state to the overall reaction, as long as the state is not so weakly bound that it dissociates thermally before being electronically quenched. Weak collision effects are allowed for using a Monte Carlo technique and an assumed exponential form for the distribution of energies transferred in collisions with a third body. For larger systems it is evident that the role of bound excited states in the low-pressure regime falls rapidly as the size of the system increases. As the high-pressure limit is approached, however, the contribution of excited states is likely to come close to that expected simply on the basis of electronic degeneracy.     

Role of excited core Rydberg states in dissociative recombination

Intermediate states formed during the dissociative recombination of molecular ions with electrons can play significant roles in determining the magnitude of the total rate coefficient. These resonances are Rydberg states of two types, that is, they can have the ground or excited states of the ion as a core. Those with the excited cores have a fundamentally different excitation mechanism than those with the ground state core. The importance of excited core states in dissociative recombination has received only limited attention in the literature. Theoretical calculations on the dissociative recombination of N2+ are reported which compare the two types of resonances. Potential curves, electronic widths, cross sections, and rate coefficients are calculated for dissociative recombination along the 2(1)Sigma(g)+ state, one of several routes for the dissociative recombination of N2+. The ground core resonances, in this example, are relatively unimportant compared to those with the excited core. Inclusion of the excited core resonances increases the rate coefficient by about a factor of 4 at room temperature, but the increase is not enough to establish 2(1)Sigma(g)+ as the dominant dissociative route.     

Cold chemistry with electronically excited Ca+ Coulomb crystals

Rate constants for chemical reactions of laser-cooled Ca(+) ions and neutral polar molecules (CH(3)F, CH(2)F(2), or CH(3)Cl) have been measured at low collision energies (/k(B)=5-243 K). Low kinetic energy ensembles of (40)Ca(+) ions are prepared through Doppler laser cooling to form "Coulomb crystals" in which the ions form a latticelike arrangement in the trapping potential. The trapped ions react with translationally cold beams of polar molecules produced by a quadrupole guide velocity selector or with room-temperature gas admitted into the vacuum chamber. Imaging of the Ca(+) ion fluorescence allows the progress of the reaction to be monitored. Product ions are sympathetically cooled into the crystal structure and are unambiguously identified through resonance-excitation mass spectrometry using just two trapped ions. Variations of the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of (40)Ca(+) involved in the laser-cooling cycle, and these are modeled by solving the optical Bloch equations for the eight-level system. Systematic variation of the steady-state populations over a series of reaction experiments allows the extraction of bimolecular rate constants for reactions of the ground state ((2)S(1/2)) and the combined excited states ((2)D(3/2) and (2)P(1/2)) of (40)Ca(+). These results are analyzed in the context of capture theories and ab initio electronic structure calculations of the reaction profiles. In each case, suppression of the ground state rate constant is explained by the presence of a submerged or real barrier on the ground state potential surface. Rate constants for the excited states are generally found to be in line with capture theories.     

Collisionless infrared multiphoton production of electronically excited parent molecules

In the absence of collisions chromyl chloride (CrO₂Cl₂) shows orange fluorescence when the parent molecule is irradiated by the focused output Time-resolved fluorescence is investigated over the pressure range 5 × 10^?4 to 15 torr. As the pressure is increased, a secondary collision-ind We interpret the collisionless part of the emission to arise from infrared transitions between the high vibrational levels of the ground state and the     

Superelastic collisions of electrons with excited Mn+

Excited Mn⁺ ions formed by electron ionization of Mn₂(CO)₁₀ are deexcited in superelastic electron-ion collisions. The ions are held in the trap of a Fourier transform ion cyclotron resonance spectrometer and subjected to bombardment by an electron beam of varying energy. The population of excited Mn⁺ ions after exposure to the beam is monitored by examining reaction of the trapped Mn⁺ ions with Cr(CO)₆. Charge transfer to form Cr(CO) ₆ ⁺ is exothermic and efficient only for excited Mn⁺. It is found that deexcitation is read if y observable for electrons with energies less than 2 eV.     

Time-dependent approach to electronically excited states of molecules with the multiconfiguration time-dependent Hartree-Fock method

In this paper the authors show how the multiconfiguration time-dependent Hartree-Fock (MCTDHF) method can be used for the calculation of electronic properties of molecules associated with the population of excited states. In contrast to other methods for correlated electron dynamics, such as configuration interaction, MCTDHF does not rely on a solution of the electronic Schrodinger equation prior to the propagation. The authors apply this approach to the calculation of vertical excitation energies, transition dipole moments, and oscillator strengths for two test molecules, lithium hydride and methane.     

Intermediate state representation approach to physical properties of electronically excited molecules

Propagator methods provide a direct approach to energies and transition moments for (generalized) electronic excitations from the ground state, but they do not usually allow one to determine excited state wave functions and properties. Using a specific intermediate state representation (ISR) concept, we here show how this restriction can be overcome in the case of the algebraic-diagrammatic construction (ADC) propagator approach. In the ISR reformulation of the theory the basic ADC secular matrix is written as a representation of the Hamiltonian (or the shifted Hamiltonian) in terms of explicitly constructable states, referred to as intermediate (or ADC) states. Similar intermediate state representations can be derived for operators other than the Hamiltonian. Together with the ADC eigenvectors, the intermediate states give rise to an explicit formulation of the excited wave functions and allow one to calculate physical properties of excited states as well as transition moments for transitions between different excited states. As for the ground-state excitation energies and transition moments, the ADC excited state properties are size consistent so that the theory is suitable for applications to large systems. The established hierarchy of higher-order [ADC(n)] approximations, corresponding to systematic truncations of the IS configuration space and the perturbation-theoretical expansions of the ISR matrix elements, can readily be extended to the excited state properties. Explicit ISR matrix elements for arbitrary one-particle operators have been derived and coded at the second-order [ADC(2)] level of theory. As a first computational test of the method we have carried out ADC(2) calculations for singlet and triplet excited state dipole moments in H(2)O and HF, where comparison to full CI results can be made. The potential of the ADC(2) method is further demonstrated in an exploratory study of the excitation energies and dipole moments of the low-lying excited states of paranitroaniline. We find that four triplet states, T1-T4, and two singlet states, S1 and S2, lie (vertically) below the prominent charge transfer (CT) excitation, S3. The dipole moment of the S3 state (17.0D) is distinctly larger than that of the corresponding T3 triplet state (11.7D).     

C3H3+ isomers: temperature dependencies of production in the H3+ reaction with allene and loss by dissociative recombination with electrons

A technique has been developed to simultaneously determine recombination rate coefficients, alpha e, and initial concentrations of ion types that coexist in a flowing afterglow plasma. This was tested using the H3(+) + allene reaction in which two different C3H3+ isomers are produced. Use of an electrostatic Langmuir probe enabled the C3H3+ isomer branching ratios for propargyl and cyclic C3H3+ from this allene reaction and their alpha e to be determined over the temperature range 172-489 K. The study showed that the cyclic C3H3+ to propargyl C3H3+ branching ratios from the allene reaction varied from 50/50 at 172 K to 18/82 at 489 K. Over this temperature range, the alpha e for both isomers change only slightly. The room temperature alpha e values for propargyl and cyclic C3H3+ are (1.15 +/- 0.2) x 10(-7) and (8.00 +/- 0.1) x 10(-7) cm3/s, respectively. The data are discussed relative to current theories and in relation to fuel-rich flame chemistry, interstellar molecular synthesis, and modeling of Titan's atmosphere.     

Coupling of the processes of dissociative recombination and scattering of slow electrons by molecular ions

A theory of dissociative recombination has been developed which makes possible an investigation of this process in connection with the problem of scattering of slow electrons by molecular ions. The simultaneous influence of direct (non-resonance) interaction and inhomogeneity of the electron continuum (due to the multichannel character of electron motion in the field of a molecular ion) is taken into account. The results obtained complement the resonance configuration interaction theory based on the Bardsley method and are important for interpreting the experimental data.     

On the electronically excited states of uracil

Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multireference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation.Our best estimates for the vertical excitation energies for the lowest singlet n --> pi* and pi --> pi* are 5.0 +/- 0.1 eV and 5.3 +/- 0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and +/- 0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A" and A' states leading to intensity borrowing by the forbidden transition.     

Formation of excited tin ion in collisions of slow electrons with SnCl2 molecules

Formation of excited tin ions in collisions of slow electrons with tin dichloride molecules was studied experimentally. Fifty excitation cross sections for SnII at an electron energy 100 eV were measured. Six optical functions for dissociative excitation were detected in the electron energy range 0–100 eV. Cross sections for direct and dissociative excitation of several lines were compared. Dissociative excitation in the spectral bands of two transitions in SnCl was also studied.     

Electron energy-dependent product state distributions in the dissociative recombination of O2+

We present product state distributions and quantum yields from the dissociative recombination reaction of O2+ in its electronic and vibrational ground states as a function of electron collision energy between 0 and 300 meV. The experiments have been performed in the heavy-ion storage ring, CRYRING, and use a cold hollow-cathode discharge source for the production of cold molecular oxygen ions. The branching fractions over the different dissociation limits show distinct oscillations while the resulting product quantum yields are largely independent of electron collision energy above 40 meV. The branching results are well reproduced assuming an isotropic dissociation process, in contrast with recent theoretical predictions.     

Static dipole polarizability of electronically excited molecules: H2(B1Σu+)

The dipole polarizability of H₂(B¹Σ_u⁺) is computed using extended Gaussian basis sets and Hartree-Fock, multiconfiguration Hartree-Fock, and configuration interaction wavefunctions. Electron correlation contributions are found to be significant (≈ 25%) with the largest contribution arising from angular correlation. With a full CI wavefunction, the components of the dipole polarizability were computed to be (in au): α_⊥ = 50 and α_| = 257.     

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.